Polypropylene provides excellent resistance to high temperature processing, such as at retort conditions of about 250.degree. F. for about 30 minutes. Polypropylene has thus found widespread use, particularly as a sealant layer in pouches which are subjected to heat treatment after the pouch is filled with product and sealed. While polypropylene has been accepted as the material of choice for use in sealant layers of retortable pouches, it has been selected more or less by necessity, as few other materials in the same general price range can provide similar high temperature processability. Likewise, for some uses such as contact with food or medical solutions, polypropylene is among the few materials which is approved for product contact. Thus, polypropylene provides a combination of desirable properties which is not found in other known materials.
Polypropylene does have some undesirable properties associated with it. Most prominent in packages is the tendency of the polypropylene to be brittle, and to be even more brittle after it has been retort processed. The degree of brittleness of a package film is most easily detected by measuring the capability of a package to withstand physically stressful treatment. Brittleness of the overall packages is a property which is a composite of the brittlenessess of the individual layers, interfacial bond strengths, the elasticity of adjacent layers and their capability to absorb and dissipate physical shock, and any overwhelming brittleness of any one layer.
The composite brittleness of retort processed packages using polypropylene is usually observed as the fraction of the filled and sealed packages which fail after being subjected to physical stress or shock.
Polyethylenes in the density range of about 0.91 to about 0.965, and linear low density polyethylene (LLDPE) have many fine and desirable properties which make them particularly suitable for many uses. Polyethylene ("PE") as used herein is defined as those polyethylenes, homopolymer and copolymer, which have a density in the range of about 0.91 to about 0.965, and LLDPE. Many PE's are stiff and rigid, able to hold shape in thick sections. PE is readily processed and formed in both the solid form and the more fluid plasticated form. It is relatively economical for many uses. In packaging applications it has been found to provide a good barrier to transmission of water vapor. And it is particularly the packaging application which is of interest to the inventors herein, while it will be seen that other applications may abound.
It would be desirable to combine layers of polypropylene and PE and, in particular, high density polyethylene (HDPE having a density greater than about 0.94), into one packaging structure to obtain the combined properties of high temperature processability, a strong heat seal and resistance to water vapor transmission at moderate thickness. The package should also have excellent resistance to physical shocks.
Polypropylene and PE layers may be combined into a multiple layer film as by adhesive lamination. Such a construction, though, has undesirable cost disadvantage of the plurality of steps necessary in its fabrication. Further, such films do tend to exhibit the brittleness of polypropylene. The brittleness may be somewhat improved upon by using a polypropylene copolymer rather than a homopolymer. While there is some improvement in performance, the general level of brittleness prevents widespread use of such structures.
If, however, an economical structure could be made which would combine the desirable properties of polypropylene and PE while improving on the ability of the package to withstand physical abuse, and to resist peeling of the heat seals, such a package may find substantial utility and acceptance. Such a structure may use blend compositions in one or more layers, and might use particular combinations of compositions, layers and positional relationships to achieve the balance of properties desired.
Thus it is an object of this invention to provide novel multiple layer films made from compositions of polymers.
It is a particular object to provide novel multiple layer films suitable for use in retortable packages, which packages have excellent resistance to rupture, and to peeling of heat seals, when subjected to shock abuse, both before and after retort processing.
A direct object is to provide closed and sealed packages using combinations of layer compositions of the invention.